1. Field of the Invention
The present invention relates to a method for bonding a plastic hydrophobic film to a metallic sheet member and to an article so produced. The invention may be of particular benefit for a bipolar cell for gas depolarized rechargeable batteries such as nickel hydrogen and zinc oxygen.
2. Discussion of the Prior Art
Bipolar batteries require a conductive planar layer between series connected cells which essentially replaces external bus bars employed in monopolar batteries. Bipolar batteries also require a means for restricting electrolyte communication between series connected cells as this completes an intrinsic short circuit when combined with the bipolar plate. In the case of nickel hydrogen batteries as taught in U.S. application for Patent, Ser. No. 08/626,992 filed Apr. 3, 1996, entitled, "Bipolar Cell Design for a Gas Depolarized Battery", this restriction is accomplished by applying an inert hydrophobic electrically insulating layer to the periphery of the bipolar plate to contain liquid electrolyte solution within the confines of the individual cell. A preferred material for this purpose, is polytetrafluoroethylene, for example, available under the trademark Teflon.RTM., manufactured by E.I. duPont de Nemours Company of Wilmington, Del.
The hydrophobic film must be bonded to the metal in order to prevent electrolyte from creeping under the layer and thereby bridging to the next series cell. The bonding between the Teflon.RTM. material and the bipolar plate must be stable in the cell environment as an unstable bond will eventually delaminate leading to an intercell electrolyte solution bridge. In practice, traditional Teflon.RTM. coating processes such as those with carbon and chromic oxide primers have not been found to be stable in the cell environment. Chemically applied coatings typically fail due to attack on the bonding interface between the Teflon.RTM. material and the metallic bipolar plate in the cell electrochemical environment.